The present disclosure relates to a catalyst basket and associated support method, and more particularly to an improved ammonia oxidation catalyst basket design that has two support grids and a method of independently supporting primary and secondary catalysts.
During the manufacture of nitric acid, significant quantities of pollutants are emitted to the atmosphere. One of the principal pollutants is the greenhouse gas nitrous oxide (N2O). Nitric acid, HNO3, is produced by oxidizing ammonia in the presence of a precious metal catalyst, “primary catalyst”. This process produces nitrogen oxides NOX. The main form of the NOx is nitrogen monoxide, NO, that is further processed with water to manufacture nitric acid, HNO3. There is N2O present in the production of the NOx and takes no further part in the chemistry of the nitric acid process and consequently is emitted to the atmosphere. The N2O can be reduced greatly by using a “secondary catalyst”. The secondary catalyst is typically installed underneath the primary catalyst. As the NOx is produced by the primary catalyst, the NOx passes over the secondary catalyst that selectively destroys the N2O.
The initial process of converting ammonia to NO for the production of HNO3 is facilitated in a piece of equipment called a converter or burner. Inside the converter there is a catalyst containment device generally referred to in the industry as an ammonia oxidation catalyst basket, along with some heat exchange equipment. Historically over the last approximately 60 years, ammonia oxidation catalyst baskets have been designed with a single support grid. The support grid is designed to hold and seal the primary catalyst in position for the ammonia oxidation process. The single support grid design is used for either a direct support of the primary catalyst or as a dual support of a fill material and the primary catalyst. For example, it is known to provide a fill material with the primary catalyst installed on top of the fill material.
With the onset of greenhouse gas reduction requirements for HNO3 production plants, several ways have been proposed to selectively eliminate the N2O. One of those ways has been to replace the fill material in the ammonia oxidation catalyst basket with a secondary abatement catalyst. This method has been proven to work very well in many plants. Unfortunately, these revisions of adding a secondary abatement catalyst to an existing ammonia oxidation catalyst basket design have resulted in process issues.
One of these process issues is known as gapping, i.e., with normal operations of a converter there will be a “gapping” of the secondary catalyst. This gapping forms at the secondary catalyst and at the ammonia oxidation catalyst basket sidewall interface. When this happens there is a preferential flow path that can cause additional issues.
Another process issue is that the abatement catalyst material has been known to compress or become compacted and in doing so reduces the height of the fill. When the height of the fill is reduced, the primary catalyst seal can be interrupted/impacted and causes still other issues.
Still another process issue results from interruptions in the process (“shutting off”) called a trip. If there are an unusual number of trips, the secondary catalyst can dome and lift the primary catalyst, and thereby interrupt a seal around the perimeter of the primary catalyst. This consequence can likewise cause additional issues.
As is evident, a need exists to address these issues in an efficient and effective manner, and at a reasonable cost that is easily incorporated into existing design parameters, and that solves these problems and others.